Optical discs currently in widespread use include a CD (Compact Disc) and a DVD (Digital Versatile Disc). Further, a BD (Blu-ray Disc) is becoming in widespread use as a next-generation optical disc, which has an increased disc capacity due to high density technology so as to store image information of higher resolution.
A recording and reproducing device using the optical disc is required to have a function to properly perform recording and reproducing on a plurality of optical discs so that not only the newest BD but also the DVD and CD owned by a user can be used.
In order to respond to a plurality of kinds of optical discs, a technology has been proposed in which a single objective lens has a plurality of focal positions. In this case, for example, objective lenses for reproducing information from three kinds of optical discs, i.e., the BD, DVD and CD, have different focal positions depending on wavelengths of laser beams to be used.
As a result, a working distance (WD) varies depending on the kind of the optical lens to be used.
Here, an object lens actuator is generally configured to generate a driving force in a focusing direction using an electromagnetic force between a coil mounted to a lens holder of a movable part and a magnet provided on a stationary part. If the working distance varies according to the kind of the optical disc to be used as described above, a reference position (i.e., a relative position with respect to the magnet of the stationary part) of the movable part in the focusing direction also varies according to the kind of the optical disc to be used.
If there are a plurality of reference positions of the movable part, it becomes difficult to keep the optical axis of the optical lens perpendicular to a recording surface of the optical disc (i.e., to suppress an angular variation in a radial direction), since a magnetic field distribution varies depending on the reference position.
Here, in order to keep the optical axis of the objective lens perpendicular to the recording surface of the optical disc, there has been proposed an objective lens driving device in which a lens holder is supported using a plurality of wires whose rigidities are different from each other (see, for example, Patent Document No. 1).
In this objective lens driving device, the lens holder is supported using four wires in total, two wires being disposed on each side of the lens holder. Further, upper two wires have higher rigidity than lower two wires, so that an upper part of the lens holder is less mobile than a lower part of the lens holder. This generates a torque in a direction to cancel a torque that causes an inclination of the lens holder, so as to suppress the angular variation of the optical axis of the objective lens.
In addition, there has been proposed another objective lens driving device of the wire-supporting type, in which midpoints of the wires are joined to side surfaces of the lens holder using damper gel, and applying positions of the damper gel differ in the left and right sides of the lens holder (see, for example, Patent Document No. 2). In this case, the inclination of the optical axis of the objective lens has been preliminarily measured, and the damper gel is applied to appropriate positions based on the measured inclination so as to suppress the angular variation of the optical axis of the objective lens.
Patent Document No. 1: Japanese Laid-Open Patent Publication No. 2003-178479 (FIGS. 11 and 13)
Patent Document No. 2: Japanese Laid-Open Patent Publication No. 2005-92949 (FIG. 13)